Skip timestamp differences for diff --no-index

We display empty diffs for files whose timestamps have changed.
Usually, refreshing the index makes those empty diffs go away.
However, when not using the index they are not very useful and
there is no option to suppress them.

This forces on the skip_stat_unmatch option for diff --no-index,
suppressing any empty diffs. This option is also used for diffs
against the index when "diff.autorefreshindex" is set, but that
option does not apply to diff --no-index.

Signed-off-by: Michael Spang <mspang@uwaterloo.ca>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff-no-index.c

@@ -247,6 +247,7 @@ void diff_no_index(struct rev_info *revs,
 	else
 		revs->diffopt.paths = argv + argc - 2;
 	revs->diffopt.nr_paths = 2;
+	revs->diffopt.skip_stat_unmatch = 1;
 
 	DIFF_OPT_SET(&revs->diffopt, EXIT_WITH_STATUS);
 	DIFF_OPT_SET(&revs->diffopt, NO_INDEX);

t/t4013-diff-various.sh

@@ -74,6 +74,10 @@ test_expect_success setup '
 	for i in 1 2; do echo $i; done >>dir/sub &&
 	git update-index file0 dir/sub &&
 
+	mkdir dir3 &&
+	cp dir/sub dir3/sub &&
+	test-chmtime +1 dir3/sub &&
+
 	git config log.showroot false &&
 	git commit --amend &&
 	git show-branch
@@ -262,6 +266,7 @@ diff --patch-with-raw -r initial..side
 diff --name-status dir2 dir
 diff --no-index --name-status dir2 dir
 diff --no-index --name-status -- dir2 dir
+diff --no-index dir dir3
 diff master master^ side
 EOF
 

t/t4013/diff.diff_--no-index_dir_dir3

@@ -0,0 +1,2 @@
+$ git diff --no-index dir dir3
+$